This is a very simple but delicious marinade. The balsamic vinegar gives the grilled steaks a deep, rich color and flavor which nicely compliments the natural flavor of the steaks. Bon Appetit, July 1998.
Puree vinegar, oil, shallots and rosemary in blender until almost smooth. pour into 13x9x2 inch glass baking dish. Add steaks; turn to coat with marinade. Cover and refrigerate at least 6 hours and up to 1 day, turning steaks occasionally.
Prepare barbecue (medium-high heat). Remove steaks from marinade; sprinkle with salt and pepper. Grill steaks to desired doneness, about 6 minutes per side for medium-rare.